Oh... that is a lot
Still, if we can imagine and treat alpha / beta releases the same way as normal releases, some can consider it all begins when the first version is posted. If you look at Minecraft for instance, it's so popular that you forget it's still in Beta stage as you play it, or that it has some bugs left. I also work on an Open Source game that had only one alpha release, and has been going on for an year now. Yet I don't feel like I'm waiting for something to happen, and feel for it like for a final project.
That said, I am wondering how unstable the merge between MG and GAE will actually be. Last time I tried them, both ran mostly bug free, except for minor issues I could notice. I never seen any true bugs in GAE, and the only one I seen in MG a few days ago was music going insane after exiting the options menu. I even played MG online, and apart from the normal lag, there were no issues there that I could notice, and it went smoothly. I'm even running MG from the SVN repository now, and not the stable releases.
So my personal feeling is that both are rather stable, although the merge between them might be a different story. Judging by that, unless the merge itself will have many new issues and instabilities, I'd imagine it to a stable point in about 6 months. But of course, I don't know as well as the admins or devs, and that's just my own feeling and 2 cents.